1. Field of the Invention
The present invention relates to monitoring of lens aberration during a semiconductor process, and more particularly, to a test key layout for precisely monitoring a 3-foil lens aberration during the fabrication of deep-trench capacitor memory devices by eliminating the COMA aberration.
2. Description of the Prior Art
The relentless drive in the integrated circuit industry toward greater packing density and higher speeds has served as the impetus for optical lithography to reduce printed image sizes. Deep-UV (DUV) lithography has been developed to scale minimum feature sizes of devices on semiconductor chips to sub-micron dimensions. However, all optical projection systems for micro-lithography depart from perfection because of various lens aberrations, especially when large image field size is combined with high numerical aperture (NA). Such aberrations have a variety of effects on lithographic imaging: shifts in the image position, image asymmetry, reduction of the process window, and the appearance of undesirable imaging artifacts. These undesirable effects are sometimes exacerbated through use of resolution enhancement techniques such as phase-shift masks or nonstandard illumination. Consequently, the lens aberration monitoring system plays an important role in the semiconductor processes.
FIG. 1 illustrates an enlarged plan view of a prior art test key layout 10 for monitoring lens aberrations that occur during the fabrication of deep trench (DT) capacitor devices. As shown in FIG. 1, the test key layout 10 comprises a plurality of DT test pairs including pair A, pair B, and pair C. Each of the pairs A, B, and C comprises a left side DT pattern 12 and a right side DT pattern 14. Typically, both of the left side DT pattern 12 and right side DT pattern 14 are rectangular shaped and, as specifically indicated, have a length L and width W. According to the prior art, pair A is disposed at a center position of the test key area 20, the pair B is arranged in 45 degree direction with respect to the pair A in the test key area 20, while the pair C is disposed in 45 degree direction with respect to the pair B. The pair A and pair C are aligned with a reference Y-axis. As seen, pair C is disposed a distance from the pair A along the ±Y-axis. In the indicated circle region 30, i.e., the area substantially surrounded by the pair A and pair B, no DT test pair is disposed therein. Typically, the lens aberration is monitored and evaluated by measuring the image distortion of the DT test pair A.
During the fabrication of DT capacitor devices, the image of the DT test pair A is affected by so-called three-foil (3-foil) aberration. However, in the meantime, the image of the DT test pair A is also affected by COMA aberration when using the same optical system. COMA is an aberration, which results in a point object being turned into a pear-shape or comet shape at the focal plane, most commonly off-axis. It is caused by unequal magnification in different zones of a lens for obliquely incident rays from an off-axis object. It is also known in the art that COMA aberration typically results in asymmetric photoresist image patterns in a photoresist layer for the originally symmetric patterns on the photo mask. The above-described prior art test key layout 10 for monitoring lens aberrations is not capable of abstracting the 3-foil aberration effect. Consequently, there is a need in this industry to provide an improved test key layout for precisely and exclusively monitoring single lens aberration effect, but not combined lens aberration effect, during the fabrication of DT capacitor devices.